I crafted an unstable portal a month ago (I've been playing for three months), and then today got my second unstable portal today in a pack. I had to try that out instantly of course, and this was the result, in the very first game (outside of tavern brawl) in which I had two unstable portals:

One unstable portal was in my first three cards, and the second or third card drawn was the second unstable portal. The first, played on turn 2 popped up Aviana for me and the second popped up Archmage Antonidus (which, along with most other legendaries, I don't have.) I had a Duplicate Spell sitting around and on turn 4, I used that, and then Archmage the next turn with a missile spell. He killed the Archmage giving me two. So I played Aviana by herself, and then on the next turn played two Archmages and two spells (maybe 3?.) I have an image with six firespells in my hand and two other spells for good measure, with both Archmages still up and a mirror spell of mine being triggered as my opponent tries to counter - but the image isn't loading.